The disproportionation of pure toluene feedstocks over molecular sieve catalysts to produce xylenes and benzene is a known phenomenon. The use of para-selective catalysts which make a xylene product containing a greater than thermodynamic equilibrium ratio of para-xylene to ortho- and meta-xylene is described, for example, in U.S. Pat. Nos. 4,117,026 and 4,097,543 of Haag and Olson.
It has been the practice to specify and use only high purity toluene as the feed for both conventional and para-selective toluene disproportionation processes. High purity toluene is usually made by extracting the aromatic compounds from a reformate or pyrolysis gas fraction and then distilling the aromatics in several steps to recover substantially pure benzene, toluene and C.sub.8 aromatics.
Toluene recovered directly from reformate or pyrolysis gasoline by distillation without prior extraction typically contains several percent close boiling nonaromatics. Nonextracted toluene has not been considered as an acceptable feed for the toluene disproportionation process because in the prior art processes the nonaromatic impurities have led to decreased benzene and xylene yields and to decreased purity for the products. Thus, the production of benzene and xylene via toluene disproportionation has required a prior aromatics extraction step which has added significantly to the cost of the final benzene and xylene products.
Catalysts can be used to selectively remove nonaromatics from reformates. For example, U.S. Pat. No. 3,849,290 describes a multi-step process to upgrade the octane rating of a naphtha gasoline blending stock. In a first step, the stock is reformed over a nonacidic platinum-type catalyst, producing a reformate containing aromatics and paraffins. The reformate is then contacted under mild hydrocracking conditions with an intermediate pore zeolite to selectively crack high boiling, low octane paraffins, e.g., C.sub.7 +. The effluent from this hydrocracking step is contacted with a small pore catalyst to selectively hydrocrack low boiling, low octane C.sub.6 - paraffins. This three step, three catalyst processing sequence yields a high octane product after the preferential removal of low octane species, but the product purity is insufficiently high for petrochemical applications.
U.S. Pat. No. 4,795,550 describes a low temperature catalytic process for removing olefinic impurities, but not paraffins or naphthenes, from an aromatic stream having a bromine index between 50 and 2000.
U.S. Pat. No. 4,150,061 describes a process whereby a fractionated pyrolysis gasoline comprising toluene, xylenes, ethylbenzenes, C.sub.7 -C.sub.10 paraffins, olefins and naphthenes are selectively hydrodealkylated and transalkylated to give ethylbenzene-lean xylenes and benzene in the presence of a catalyst comprising a tungsten/molybdenum component (WO.sub.3 --MoO.sub.3) and an acidic component of 60 wt % mordenite and 40 wt % catalytically active alumina. The product is then distilled to provide benzene and xylene streams of unknown purity and a toluene stream for recycle.
U.S. Pat. No. 4,861,932 describes a process for producing gasoline blending stocks in which nonaromatic C.sub.2 -C.sub.12 paraffins are converted to a mixture of higher octane aromatics and alkylaromatics by first contacting the paraffins with a noble metal/low acidity catalyst. The effluent is then contacted with an acidic catalyst based on a zeolite such as ZSM-5 with a metal such as gallium (Ga). Although the Ga/ZSM-5 catalyst is known to have a high aromatic selectivity, the product purity is insufficiently high for petrochemical applications.
A large number of molecular sieves are known to have use as catalysts in various hydrocarbon conversion reactions such as disproportionation, aromatization including reforming, catalytic cracking, hydrocracking, dehydrocyclization, isomerization and dewaxing. Typical intermediate pore size molecular sieves of this nature include ZSM-5, silicalite, generally considered to be a high silica to alumina ratio form of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, SSZ-32, SAPO-11, SAPO-31, SAPO-41, and the like. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 are described in U.S. Pat. Nos. 3,700,585; 3,894,938; 3,849,290; 3,950,241; 4,032,431; 4,141,859 4,176,050; 4,181,598; 4,222,855; 4,229,282; and 4,247,388 and in British Patent 1,469,345. However, the use of such catalysts, particularly acidic and para-selective forms of such catalysts, for the production of high purity benzene and para-xylene enriched xylene products from a C.sub.7 -rich aromatization product, is not known and has generally not been contemplated.
It would be highly advantageous if one could use refinery streams, for example, aromatizer product streams which had at most been distilled so as to have a reasonably high toluene content, but which still had a significant amount of impurities which boiled in the toluene range, in a process which would selectively produce para-xylene enriched xylene along with benzene, and if desired chemically pure toluene as well and would at the same time convert the impurities to products which could be readily separated from the para-xylene enriched xylene and the benzene and from toluene, if desired, by simple distillation.